Make it possible to call a VRF API implemented in Algorand's libsodium

[torao]

Use libsodium (used by Algorand) for VRF. It's an extension of the original security library jedisc1/libsodium to include VRF functionality.

The libsodium is implemented in C and should be called via cgo from Tendermint.

The header file sodium.h is required to use constants and functions in libsodium from Golang. Therefore, I'll place github.com/algorithm/libsodium on crypto/vrf/internal/vrf/libsodium
(with git submodule), according to other packages, such as crypto/secp256k, which is part of Tendermint.

Additionally, we must also compile and install the library before we use it.

[torao]

This modification will require libsodium to be installed before building Tendermint, as follows:

$ cd crypto/vrf/internal/vrf/libsodium
$ ./autoconf.sh
$ ./configure
$ make && make check
$ sudo make install

The symbols crypto_vrf_* will then be visible on libsodium.a,

$ nm /usr/local/lib/libsodium.a | grep vrf
/usr/local/lib/libsodium.a(libsodium_la-crypto_vrf.o):
U _crypto_vrf_ietfdraft03_is_valid_key
U _crypto_vrf_ietfdraft03_keypair
...

and can be called from golang.

package main

/*
#cgo CFLAGS: -I./internal/vrf/libsodium/src/libsodium/include
#cgo LDFLAGS: -lsodium
#include <sodium.h>
*/
import "C"
import "fmt"

func main(){
  fmt.Printf("public key bytes: %d\n", C.crypto_vrf_publickeybytes())
}

[torao]

push on branch feature/integrate_libsodium.

[torao]

To be taken over to PR # 4.